Along with advances toward higher frequencies and miniaturization of information and telecommunications devices such as portable telephones, electronic components such as filters and oscillators used in these devices are also undergoing advances toward higher frequencies and miniaturization. As shown in FIG. 11 for example, filters and the like are being used in which a bulk acoustic wave (BAW) resonator is used. Ordinarily, piezoelectric materials are used to produce BAWs, and the resonance frequency of the resonator is determined by the dimensions of the piezoelectric material, such as its thickness for example. Examples of piezoelectric materials include piezoelectric single crystals such as lithium tantalate and lithium niobate, piezoelectric ceramics such as PZT (lead zirconate titanate), and piezoelectric thin films such as ZnO (zinc oxide) and AlN (aluminum nitride), but piezoelectric single crystals and piezoelectric thin films are most commonly used in products that have exacting specifications in terms of frequency precision such as those used in mobile communications and the like. It is common to use piezoelectric single crystals when the range of frequencies that is used is not greater than several hundreds of MHz, and to use piezoelectric thin films for frequency ranges greater than that. This is because the resonance frequency of the above-mentioned BAW resonators is determined by the shape of the element. For example, the resonance frequencies for BAW wave modes such as thickness longitudinal oscillation and thickness shear oscillation used in filters and the like are determined by the thickness of piezoelectric material. Accordingly, the precision of the thickness of the piezoelectric material corresponds to the precision of the component's frequency, and thus thickness adjustments are an important issue. Adjustments of frequencies, that is, plate thicknesses, can be divided into rough adjustments and fine adjustments. Rough adjustments are performed by using grinding for piezoelectric single crystals, and by controlling the film formation process for piezoelectric thin films. Fine adjustments supplement rough adjustments, and are an essential technology for improving component performance and improving yield. Furthermore, with a piezoelectric thin film that will be discussed below, miniaturization is possible by forming a component on a single chip, and this is also an important technology for being able to form resonators that have different resonance frequencies on the same substrate. The most commonly used fine adjustment method is an adjustment vapor deposition method using the mass loading effect. This is a method in which a frequency adjustment is carried out by depositing on the excitation electrode, with which the BAW resonator is provided, a further electrode for frequency shifting. It should be noted that resonance frequency is reduced by mass loading.
Furthermore, as an example of a different method, a method is proposed such as that in JP 2002-359539A. As shown in FIG. 18, an electrode 202 and a piezoelectric material 201 are arranged on a substrate 206. Cavities are provided in the substrate 206 corresponding to portions of the piezoelectric material 201 that perform elastic oscillation. Moreover, there are two resonators present. On the piezoelectric material 201 there are a resonator provided with a surface electrode 205 and a resonator provided with a surface electrode 203. By including an oxidized conductor portion 204 as a portion of the surface electrode 203, the mass of the surface electrode is increased and the frequency is reduced.
As an example of a method in which the frequency is raised and fine adjustments are carried out, a method as in JP 2002-359534A has been proposed. This is a method in which a portion of a surface electrode 207 is removed by etching as shown in FIG. 19, such that its resonance frequency is made higher than that of the resonator provided on a portion of a surface electrode 209. As another approach, a method as in JP 2002-335141A has been proposed. This is a method in which a surface loading layer 211 is arranged on a surface electrode 210 as shown in FIG. 20, and then the surface loading layer 211 is etched so that the resonance frequency of the resonator becomes a desired frequency.
However, with methods such as adjustment vapor deposition methods using mass loading or the method according to JP 2002-359539A, the frequency only can be adjusted in a downward direction. Furthermore, with the method according to JP 2002-359539A, the resistance of the surface electrode is increased by oxidization, and resonator characteristics deteriorate, such as the frequency acuteness Q value for example. A method is available in which the thickness of the oxidation layer is reduced as a method of reducing such deterioration in characteristics, but this involves narrowing the frequency adjustment width. Furthermore, there is also a method in which the thickness of the surface electrode 203 is increased so that the relative thickness of the oxidation layer 204 is reduced, but this reduces the degree of freedom in design since the surface electrode 203 is an important design parameter. Furthermore, this creates the problem of inviting fluctuation in parameters such as the electromechanical coupling coefficient.
With the method according to JP 2002-359534A in which frequencies are adjusted in an upward direction, the resistance of the surface electrode 207 is increased by reducing the thickness of the surface electrode 207, thus worsening the Q value. Furthermore, the problem also arises that a modified layer occurs in a portion of the surface electrode 207 due to etching and the resistance is similarly raised, thus worsening the Q value. Furthermore, empirically, the problem also arises that spurious signals, which are unwanted oscillations, tend to be produced when performing etching on portions of excitation portions of electrodes.
With the method according to JP 2002-335141A, a process by which a surface loading layer 211 is arranged becomes necessary, thus leading to increased costs. Since the surface loading layer 211 is arranged on a surface layer electrode 210, which is an excitation electrode, the problem arises in that it becomes easier for spurious signals to be produced.